1. Field of the Invention
The present invention relates to a diaphragm for a speaker and a method of producing the same, and, more particularly, it relates to a speaker diaphragm improved in the material of which it is formed and in the bound or interlaced condition of said material in order to reproduce sound with high fidelity, and a method of producing the same.
2. Description of the Prior Art
In order to reproduce sound with high fidelity by a speaker, it is desirable that the diaphragm, which is the vibrating body of the speaker, be capable of vibrating in as wide a frequency range as possible. To this end, it is advantageous for the diaphragm to be light in weight and rigid. More specifically, the greater the ratio E/.rho., where E is Young's modulous and .rho. is density, the more advantageous. This is because the higher the rate of propagation of sound, .sqroot.E/.rho., through the diaphragm material, the wider will be the frequency range of vibration.
For this reason, many attempts to increase the rate of propagation of sound, .sqroot.E/.rho., through the diaphragm material by screen-processing a mixture of a highly elastic fiber, e.g., carbon fiber, metal fiber or inorganic fiber and a natural fiber, that is, pulp, in a manner similar to that employed for making paper, have heretofore been made. However, satisfactory results have not been obtained. This is because the existing diaphragms produced in this way suffer detrioration of the speaker frequency characteristics, especially a decrease in the output sound pressure level in the treble, or high, frequency range.
A prior art reference of interest in relation to this invention may be found in Japanese Patent Publication No. 23505/1970, "Radio Cone", published on Aug. 7, 1970. Therein is disclosed a speaker cone produced by cutting carbon filaments into short fibers, mixing the latter with pulp having a binder, such as polyvinyl acetate resin, added thereto, and screen-processing said mixture in a manner similar to that employed for making paper. Since the carbon fiber use therein is, in itself, highly elastic, it might seem to be capable of increasing the rate of propagation of sound through the diaphragm material. On the contrary, since it is highly elastic, the fibers rub each other during vibration, so that the interlaced carbon fibers will become loose in prolonged use, thus leading to deterioration of the characteristics. More specifically, the use of a binder, such as polyvinyl acetate resin used therein, alone fails to produce a sufficient force for binding fibers together to provide a structure having the fibers firmly bound together. This also results in failing to provide sufficient rigidity required for a diaphragm. Therefore, the speaker cone disclosed therein, as such, can hardly be used as a speaker diaphragm and there would be much room for improvement.
Another prior art publication of interest in relation to this invention may be found in Japanese Laying-Open No. 63912/1975, "Diaphragm for Electro-Acoustic Transducer" laid open on May 30, 1975. Therein is disclosed a diaphragm comprising a layer-like body of electrically conductive fiber, such as carbon fiber or metal fiber, having at least one surface thereof formed with a metal layer by electroplating. According to this disclosure, since the surface of the fiber assembly constituting the diaphragm is plated with a metal layer, the binding of fibers at least on the surface of the diaphragm can be attained. But in the interior of the diaphragm, the binding of fibers is still weak, so that the resulting structure has its fibers loosely bound together as a whole. Further, the rigidity in the interior of the diaphragm is also insufficient. This is due to the fact that the metal plating layer is formed by electroplating. The art disclosed therein also has the disadvantages inherent in electroplating. One of the disadvantages is low productivity. More specifically, the necessity of placing electrodes opposed to a diaphragm to be plated limits the number of diaphragms which can be plated in each operation. Further, in order to form a plating surface of good quality, it is necessary to prepare an electrode extending along the shape of the cone-shaped diaphragm. Even with this, however, it is impossible to preclude a phenomenon in which a thicker plating layer is formed on the peripheral edge of the diaphragm. Another disadvantage of electroplating is that a restriction of being electrically conductive is imposed on the material to be plated. Therefore, the freedom of selection of materials for making diaphragms is decreased.